To prepare filling: Heat oil and sear the chicken over high heat until browned. The chicken should not be fully cooked. Remove from pan and set aside.
In the same pan, add butter and cook mushrooms over high heat until golden brown. Remove from pan and set aside with the chicken.
In the same pan, fry the shallots over low heat until translucent. Add cream and bring to boil. Add mustard, chicken and mushrooms and season to taste. Dish into the pie dish.
To assemble & bake: Roll out the thawed puff pastry and place over the top of the pie dish. Brush with egg wash. Bake in a preheated at 180ºC for 15 minutes or until pastry is crisp and golden.
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